Metallic electrical conductor for evacuated and gas-filled fused quartz devices



Jan. 8v, 1935. w T ANDERSON, JR 1,986,784

METALLIC ELETRIICAL CONDUCTOR FOR EVACUATED AND GAS FILLED FUSED QUARTZ DEVICES Filed March 8, 1932 IN V EN TOR.

A TT ORNEY Patented A Jan. 8, 1935 PATENTI OFFICE 1,986,784 y METALLIC ELECTRICAL CONDUCTOR Fon EVACUATED AND QUARTZ DEVICES GAS -FILLED FU SED William T. Anderson, vJr., Newark, N. J., assignor. to Hanovia Chemical and Manufacturing Company, Newark, N. J.,

Jersey a corporation of New Application March 8,1932, Serial No. 597,462

2 Claim. (Cl. 176-50) This invention relates to evacuated and gas' lled fused quartz devices, and more particularly to-means for making gas tightseals between such ,devices and metallic electrical conductors.

Many attempts have been made to seal metallic conductors into fused quartz for the production and .attainment of a gastight seal. However,

. numerous difficulties have been encountered.

For example, fused quartz has an extremely small coefficient of thermal expansion, 0.6 106; and an alloy of nickel and iron, known as invar, has a coeicient of thermal expansion approximating this value, and might possibly be fused into quartz to make a gastight seal if it were not for the fact that the invar metal melts at a temperature below the fusion and working temperature of' quartz. Invar metal is fluid before the quartz can be worked, and as a result it is not possible to attain a seal between the two. No other suitable substances of similar expansion are known. Tungsten and molybdenum with expansion coeicients of approximately 4.2)410-8 are the closest of the metals, and these two substances, in addition, have melting points substantially above the working temperature for quartz.

Expedients which have heretofore been used to fuse tungsten, molybdenum and similar metals to quartz are discussed in U. S. Patent No. 1,909,797, granted to me on May 16, 1933 for Method of forming electrical lead-ins for fused quartz devices.

Attempts have been made to fuse tungsten and molybdenum into quartz to produce vacuum and gastight seals. Thus, in U. S. Patent No. 1,608,612, November 30, 1926 is described a special treatment of molybdenum which enables it to be fused directly into the quartz.

It is an object of my invention to provide a vacuum tight and gas tight metallic conductor which is suitable for usewith quartz mercury arcs and quartz gas filled devices, which is inexpensive to manufacture, very durable, and capable of withstanding all temperature conditions to which r such, devices are subjected.

A further object is the provision of a metallic conductor for evacuated and gas lled fused quartz devices, which can be manufactured at a considerable less cost than conductors heretofore used in such devices, with an inappreciable percentage of failures.

These and other advantageous objects, which will later appear, vare accomplished by the simple and practical construction and arrangement of parts hereinafter described and exhibited in the accompanying drawing, forming part hereof, and in which:

The Figure l is a sectional view of my metallic conductor. Fig. 2 shows the manner in which the conductor is attached to a gas filled quartz electric device.

Referring to the figures, the conductor for fused quartz vessels consists of a metal core 3, which has a melting point higher than the fusing and working temperature of fused quartz, namely 1700 degrees centigrade, and a coeiiicient of thermal expansion less than 9.0 106, a fused quartz sleeve 4, about the metal core, and a suitable vacuum sealing compound 5 occupying a portion of the interfaces between the metal and the quartz. v

Tungsten and molybdenum metals which melt at 3400 degrees centigrade and 2535 degrees centigrade, and have thermal expansion coefficients of about 4.2 105, are suitable for use in ny invention. The quartz sleeve 4, that encloses the metal core 3 must t snugly but not tightly, and it must fit more snugly on the end A of the sleeve towards and adjacent to the quartz mercury arc or quartz system than on the end B where electrical connection to the outside is to be made. The sleeve supports? the metal core 3, but the metal core may be turned or moved if sufficient pressure be applied. The sealing compound is quite critical as to physical characteristics and must comply closely to certain requirements hereinafter described if a practical and enduring seal is to be obtained.

The compound 5 must have a high degree of elasticity and compressibility. Quartz mercury arcs which employ my invention are shipped to all parts of the world and must withstand during shipment temperatures ranging from the subzero of winter to the high temperatures of the tropics. In addition, the operating temperature of these sealed metallic-conductors very constantly between normal room temperature and 100 and more degrees centigrade. The compound lls in part the interface between the metal core andthe quartz sleeve. The core 3 expands with an increase in temperature more rapidly than the quartz sleeve 4 and therefore fits the quartz sleeve 4 more snugly at a high temperature than at a lower one. Similarly, as the conductor 3 cools the metal contracts more rapidly than the quartz sleeve 4 and the space between the two is increased. The compound 5 must therefore adjust itself to this change. It must, therefore, be compressed as the conductor warms, and expand elastlcally as the conductor compression and expansion of the compound must be repeated many thousands of times during the life of the electrical device.

Another vessential is that the compound adhere very firmly to both the metal and the quartz,

and remain iirmly attached under all conditions of service. If it does not do so, moisture and air will pass between the compound and the other components of the conductor and destroy the vacuum and gas tight device to which the conductor was attached. f

The compound must not absorb moisture and gas, and it must not pass water and gas from the atmosphere into the electrical device, and neither must it pass gas, if it be gas filled, from the device to the atmosphere.

The compound should not melt at too high a temperature and its changes in fluidity should be gradual. I have found that a compound for my conductor should, at 100 degrees centigrade, have ya viscosity coeicient of the order of 20,000 dyneseconds per square centimeter, and the change in viscosity with temperature should be slow, and accompanied with a relatively small volume change. As a result the change from solid to uid condition should pass through a plastic state whichl permits opportunity for readjustment within the seal .and without danger of rupture.

Since my conductor is designed for employment onquartz mercury arcs and quartz gas-filled devices which are either highly evacuated or contain gases under low pressure it is important that the compound used for the seal have a composition s uch that none of its constituents can pass into the quartz system' and interfere with the proper function of the system. The compound should vhave a negligible vapor pressure under all operating conditions.

I have foundthat a compound which complies with the above specifications may be prepared from a mixture f the higher hydro-carbons found -in coal tar (pitch) and the carbonaceous materials resulting from the distillation of petroleum. The chemical compositions of these materials are highly variable, depending in part on their source land in part on subsequent treatment. All specimens do not prove to be suitable for my invention, and I have found it necessary to evaluate them and determine the percent mixtureby trial and test. 'I'hat is, the hydro-carbons and carbonaceous materials are mixed until a compound is obtained having the viscosity characteristics specified above.

I have also found that there are available a few commercial hydro-carbon mixtures which may be adapted to my invention. Since these also vary, not all samples of the same trade name have proved satisfactory.

vIn making a conductor in accordance with my -invention,the metal core 3, such as tungsten or molybdenum, is placed within4 the fused quartz sleeve 4. It is desirable to have'the sleeve rlt closely; thus if the metal core be in the form of a wire 1 millimeter in diameter, the bore of the sleeve can conveniently be 1.25 millimeters.

One ,end of the quartz sleeve 4 is closed by fusion before the metal is introduced or by means of a suitable stopper; the first method, fusion, being preferable. 'I he other end of the sleeve ls attached to a'n evacuation pump, and the sleeve is evacuated. The function of the vacuum is two fold. The removal of air is necessary in order that the metal will not be oxidized during subsequent heat treatment. The reduction in gas pressure within the quartz sleeve assists in the process of forming the quartz about the metal. This will now be described.

`The evacuated quartz sleeve containing the metal core is heated to the working temperature vof fused quartz. This may be `accomplished by flames or electrically. 'I'he quartz must be brought to a temperature of about 1650 degrees centigrade. The portion of the sleeve which will be on the external, that is, the atmospheric side of the conductor is heated rst. 'Ihe quartz softens and settles about the metal. 'Ihe heating is continuedalong the sleeve and the intensity of the'heating is increased as the end A of the sleeve, adjacent tothe body of the quartz electric device, is approached, the vquartz electric device being not shown in the drawing;

The conductor is then cooled to room temperature (24 degrees C.). If the heat treatment has been properly applied, the quartz will be free of cracks. The metal as itA cools will contract more rapidly and to a greater extent than the quartz, and hence, will pull awayfrom the quartz. The gap produced between the metal and the quartz will be greatest on the external end 3 of the conductor and the least on the end A to be attached to the gas-nlled electrical device.

The conductor is then removed from the evacuating system, the closed end opened, and the volatilized metal, if there be any, cleaned out by acid or gentle glowing.

The conductor may then be attached to the quartz electrical device. It is also practical in some instances to shrink the quartz sleeve about the metal while attached to the main body of the quartz electrical device.

The gap between the metal core and the quartz sleeve at the end B is then tllled with a compound as above described, which hermetically seals the metal to the quartz under all norm'al and operating conditions.

Thefforegoing disclosure is to be regarded as descriptive and illustrative only, and not as restrictive or limitative of the invention, of which obviously an embodiment may be constructed in' cluding many modifications without departingv from the general scope herein indicated and denoted in the appended claims.

In Fig. 2 is shown the manner. in which the metallic conductor is attached to a gas lled quartz electrical device. It be seen that the sleeve 4 is fused to a U-shaped tube 6, which in turn is connected to and forms a part of the gas filled electrical device 7. The arrangement above described lessens the chances of breakage and damage of the quartz elements due to blows from the pools of mercury caused by jostling while being transported.

Having thus described my invention, what I clatim as new and desire to secure by Letters Paten ,'is:

1. In a lead-in conductor for use with quartz mercury electric devices, having a straightrelatively long metal core, a fused quartz sleeve sur-- roundingthe core, s^id sleeve tightly fitting the core at' one end and slightly spaced from the core throughout its remaining length, said'spacing being gradually increased from the tight-fitting end to the other end of the sleeve, said spacing at'said other end being less than the diameter of. the core and a compound consisting of a mixture of hydrocarbons having a viscosity o! approximately 20,000 dyne-seconds per square centimeter at 100 degrees centigrade, said compound being in the space between the sleeve and the core. providing a moisture and gas tight seal between the sleeve and core.

2. In a-lead-in conductor for use with vapor electric devices, having a straight relatively long metal core, a sleeve of vitreous material sur- 10 rounding the core, said sleeve tightly fitting the core at one end and slightly spaced from the core throughout its remaining length, said spacing being gradually increased from the tight ntting end to the other end of the sleeve, said spacing at said other end being less than the diameter of the core, and a hydrocarbon compound in the space between the sleeve and the core, providing a moisture and gas tight seal between the sleeve and core.

WILLIAM T. ANDERSON. Jl. 10 

